linux-kernel - Re: [tip: x86/cpu] x86/cpu/intel: Replace PAT erratum model/family magic numbers with symbolic IFM references

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Message-ID: <c9f18ae0-8239-495a-abc2-d6538fbe5f5e@intel.com>
Date: Tue, 3 Sep 2024 11:46:52 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar
 <mingo@...nel.org>, Len Brown <len.brown@...el.com>, x86@...nel.org,
 Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [tip: x86/cpu] x86/cpu/intel: Replace PAT erratum model/family
 magic numbers with symbolic IFM references

On 9/3/24 02:32, tip-bot2 for Dave Hansen wrote:
> -	if (c->x86 == 6 && c->x86_model < 15)
> +	if (c->x86_vfm >= INTEL_PENTIUM_PRO &&
> +	    c->x86_vfm <= INTEL_CORE_YONAH)
>  		clear_cpu_cap(c, X86_FEATURE_PAT);

Andy Cooper did point out that there is a theoretical behavioral change
here with c->x86_model==0.  There is a reference to the existence of
such a beast on at least on random web page[1] on the Internet as "P6
A-step".

But the SDM neither confirms nor denies that such a model ever existed.
If the SDM can't be bothered to acknowledge its existence, Linux
probably shouldn't either.

Either way, we're talking about a 32-bit CPU that's almost 30 years old
and was probably pre-production anyway.

I'm fine with the patch as-is.

1. https://www.sandpile.org/x86/cpuid.htm